In the operating range of an internal combustion engine, a conventional three-way catalytic converter may no longer fulfill the requirements for nitrogen-oxide conversion when the fuel-air mixture is lean (lambda>1). NOx storage catalysts, which store nitrogen oxides emitted during lean-combustion operation, are used in this case. Stored nitrates are released and reduced to nitrogen by operating the engine in a rich range (lambda<1). The use of an NOx storage catalyst in this context is discussed, for example, in European Patent No. 0 560 991.
Legislative requirements provide for an on-board diagnosis of motor-vehicle components relevant to pollutant emissions, such as catalytic converters. In this connection, it is discussed, e.g., in German Published Patent Application No. 24 44 334 that one may use the output signals of an oxygen-sensitive exhaust-gas analyzer probe positioned in front of the catalyst and one positioned behind the catalyst to assess a three-way catalytic converter. The conventional method is based on the capability of a functioning three-way catalytic converter to store oxygen. In this context, German Published Patent Application No. 24 44 334 discusses the changing of the fuel-air mixture composition from lambda=0.95 (rich, fuel-rich mixture; oxygen deficiency) to lambda=1.05 (lean, fuel-poor mixture; excess oxygen). The exhaust-gas sensor arranged in front of the catalytic converter reacts to the change in the fuel-air mixture composition with almost no delay. Due to the prevailing oxygen deficiency in the exhaust gas at lambda=0.95, the oxygen storage locations of the catalyst are initially not occupied. After the switchover to lean-combustion operation (excess oxygen) in front of the catalyst, the oxygen storage locations are successively occupied. Therefore, after the composition of the mixture is changed, the oxygen deficiency initially continues in back of the catalyst. After a period of time dependent on the oxygen storage capability of the catalyst, an excess of oxygen also occurs behind the catalyst, triggering a change in the signal of the rear exhaust-gas sensor. The time delay, i.e. the phase shift between the reactions of the two exhaust-gas sensors, decreases with decreasing oxygen storage capability of the catalyst and may therefore be used to assess the oxygen storage capability for diagnosing the catalyst.
This conventional method may not be easily applied to a catalytic converter, which, in addition to being capable of storing oxygen, is also capable of storing nitrogen oxides (NOx). Such catalytic converters may normally store nitrogen oxides as well, when their oxygen storage capability is already exhausted, and an exhaust-gas sensor arranged behind the catalytic converter indicates an excess of oxygen. In the case of NOx storage catalysts, the time delay between the reactions of the two exhaust-gas sensors after the mixture composition is changed from a rich to a lean mixture therefore does not supply any information about their NOx storage capability.
Internal combustion engines having direct gasoline injection may provide reduced carbon dioxide (CO2) emissions. Since these engines are predominantly operated with a lean fuel-air mixture, they are provided with a nitrogen-oxide (NOx) storage catalytic converter, which stores NOx emissions formed in the lean mixture phase, and is freed from the stored nitrogen oxides by operating the engine with a rich mixture. Since engines having direct gasoline injection are also operated at lambda=1, the NOx storage catalytic converters are, generally, also capable of storing oxygen. A conventional three-way catalyst may be used, for example, to store oxygen.
Since the capability of a catalytic converter to store nitrogen oxides is limited, the catalyst is regenerated from time to time. The times for beginning and ending the regeneration are important with regard to the emissions discharged in back of the catalyst into the environment. During lean operation of an internal combustion engine, the NOx storage catalyst is filled with nitrogen oxides and the three-way catalyst is filled with oxygen. The beginning of the regeneration phase is stipulated by an NOx storage model. This models the amount of nitrogen oxides introduced into the NOx storage catalyst and therefore models its NOx level. If the modeled variable exceeds a specifiable threshold, then a regeneration phase (operation of the internal combustion engine with a rich mixture) is initiated.
It has been shown that, when the engine is operated with a rich mixture, the NOx storage catalyst is initially emptied before the oxygen reservoir is completely emptied. When regeneration is performed so long that the oxygen reservoir is completely emptied, then this creates high hydrocarbon (HC) emissions, since the rich mixture is no longer buffered by the oxygen reservoir. For this reason, it is important to just regenerate the catalytic converter long enough to empty the NOx storage device, but the oxygen reservoir should not yet be emptied. This constitutes regeneration that is optimal with respect to hydrocarbons.
German Published Patent Application No. 198 01 625 discusses that the regeneration phase may be ended when an output signal, e.g., an output voltage, of the exhaust-gas sensor arranged behind the catalytic converter exceeds a specifiable threshold value. It is then assumed that the NOx store is completely emptied but the oxygen store is not completely emptied. However, this conventional method provides that the output signal of the exhaust-gas sensor is subject to certain fluctuations and may therefore exceed the predetermined threshold value at various times. The fluctuations of the output signal are caused by manufacturing tolerances and temperature fluctuations of the exhaust-gas sensor.
It is an object of the present invention to be able to detect the end of a regeneration phase of the catalytic converter as safely and reliably as possible.
In order to achieve this object, the method according to the present invention involves evaluating the gradient of an output signal of the exhaust-gas sensor.